The invention relates to a control system known for instance from British Pat. No. 12 35 891. While in former times water and air heaters in private homes as well as in central heating systems were generally operated with gas of about the same heating value, today gases are offered such as manufactured gas, natural gas and liquified gas which differ from each other with respect to their heating value as well as with respect to the required amount of combustion air for achieving complete combustion. As characterizing value for the heat content and the density of the gas, usually the so-called Wobbe index is used which for the various kinds of gases has essentially different values. For optimum use of the fuel it is desired to reach almost complete combustion with little excess of air. While for burning 1 m.sup.3 of manufactured gas about 3 to 4 m.sup.3 of air are required, for burning 1 m.sup.3 natural gas one needs about 8 m.sup.3 and for burning 1 m.sup.3 butane about 30 m.sup. 3 are necessary.
Since burners and the associated control equipment in most cases are adjusted to a predetermined kind of gas, and, if necessary, by particular service measures can be changed to another kind of gas, the gas supply companies have tried to remove the difficulties for their customers arising from the supply of different kinds of gases by mixing the fuel gas with an auxiliary gas, for instance air or nitrogen such that independently from the kind of gas, the gas supply network is supplied with a gas having about the same Wobbe index. Such mixing apparatus, however, are expensive and require a strong monitoring with respect to safety requirements in order to prevent that without the supply of additional combustion air, a burnable gas mixture is flowing through the supply network or can be formed in the piping.
The invention therefore strives for a water or air heater which can be heated by gas of different Wobbe index. This object is achieved by a control system of the kind as characterized in claim 1. By the invention it is possible to operate the burner in an optimized manner, that means with complete combustion of the fuel gas with low excess of air and therewith to use the supplied fuel as well as possible in spite of different heating values of the gases.
The use of oxygen or carbon dioxide sensors for monitoring the exhaust gases of combustion motors and the exhaust gas of power stations and industrial plants is known as such. The invention accomplishes by the use of such known sensors a control system which allows to operate the water or air heater with different kinds of gases and simultaneously makes optimum use of the fuel gas. Preferred embodiments of the invention are described in the subclaims.